1. Field of the Invention
This document relates to a display apparatus, and more particularly, to a liquid crystal display device and an array substrate for that device.
2. Discussion of the Related Art
In related art liquid crystal display devices, a liquid crystal layer having an anisotropic dielectric constant characteristic is formed between a color filter substrate and an array substrate. When an electric field is applied to a liquid crystal material, molecular arrangement of the liquid crystal material changes by the intensity of an electric field to transmit light through the color filer substrate. An amount of light transmit through the color filter substrate is controlled, thereby displaying a desired image. In general, the related art liquid crystal display devices including a thin film transistor-liquid crystal display (TFT-LCD) device uses a TFT as a switching device.
The related art liquid crystal display devices are not the self-emissive type display device. Accordingly, a separate external light source is necessary. The related art liquid crystal display devices are classified into a backlit type liquid crystal display device (backlit type) and a reflective type liquid crystal display device (reflective type), depending on a kind of light source used. In the related art backlit type, a backlight unit is assembled at a rear surface of the backlit type and the light is generated from the backlight unit. In the related art reflective type, external light is reflected on a color filter substrate.
Recently, transflective type liquid crystal display devices (transflective type) have been developed. In the transflective type, one color pixel includes a reflection area and a transmission area and a length of an optical path in the reflection area is twice as long as that of the transmission area. The transflective type reduces unnecessary power consumption caused by the backlight unit, displays images even when there is no external light source, and switches an operation mode freely between a reflection mode or a transmission mode. In addition, the light reflected on the transflective type travels through a liquid crystal layer positioned between a color filter substrate and an array substrate twice as fast as that of the related art liquid crystal display device.
To reduce a retardation difference of light caused by the difference in the optical paths, a cell gap in the reflection area is set to be greater than a cell gap in the transmission area. Accordingly, a phase retardation of the reflection area and a phase retardation of the transmission area are maintained at an equal level, thereby continuously maintains a luminance in the transflective type when running in the reflection mode and the transmission mode.
In the transflective type, an upper retardation film is provided on an upper portion of the color filter substrate and a second retardation film is positioned on a second portion of the array substrate to compensate the retardation difference. In general, the transflective type uses a retardation film to compensate difference in the transmittance-voltage curves among the transmission areas of red, green and blue color pixels. Each transmission area includes unique transmittance-voltage characteristic. In this case, the red, green and blue color pixels each have a different black driving voltage (reflectivity=0) in red, green and blue wavelength bands (red≈450 nm, green≈550 nm, and blue≈650 nm).
To obtain the retardation difference in a visible light band (about 300-700 nm) including all the red, green and blue wavelength bands, a wideband retardation film having a multi-layered structure is necessary. The wideband retardation film allows reflectivity-voltage characteristic curves of the reflection areas in red, green and blue color pixels to be equal to each other. In addition, the transmittance-voltage characteristic curves of the transmission areas in each color pixel to be equal to one another. However, since the wideband retardation film is thick due to its multi-layered structure, it is difficult to manufacture the liquid crystal display device thinner and lighter at a lower cost.